The invention relates to aqueous formulations comprising organic binders having a siloxane content and inorganic nanoparticles, a process for the preparation thereof and the use thereof for the preparation of aqueous coating compositions.
In modern lacquering concepts of automobiles, clear lacquer plays an essential role as the top layer. In this context, in addition to aesthetic effects, such as high gloss and transparency, the protective function of the clear lacquer is an essential aspect. The clear lacquer protects the underlying lacquer layers against external influences, such as sunlight, water, solvents and aggressive chemicals, and last but not least against mechanical stress. The scratch resistance of automobile clear lacquers is therefore still an essential criterion for the quality of an automobile clear lacquer.
Nanoparticles in polymeric coatings can improve properties such as scratch resistance, UV protection or conductivity in a targeted manner. Control of the surface modification and dispersion of the nanoparticles determines the required transparent appearance of the coatings and the properties thereof.
Various approaches have been pursued in the past for introduction of the nanoparticles into coating composition formulations. In this context, the particles can be mixed directly into the resin or curing agent component or into the coating composition ready for application. In aqueous systems there is the possibility of dispersing the particles in the aqueous phase. The in situ preparation of the particles in one of the binder components and adaptation of the surface to either the resin or the curing agent component have furthermore been described.
From the practical point of view, it is advantageous to disperse the nanoparticles as stable masterbatches in one of the components, so that a long-term storage stability and a simple ease of handling in the formulation of lacquers is ensured. In the end use, the nanoparticles must likewise be readily dispersible in a finely divided manner, so that advantageous properties such as transparency, scratch resistance or conductivity result.
In practice, the nanoparticles are conventionally dispersed into the resin component, into the aqueous phase or into the finished mixture of curing agent and resin shortly before curing. As a rule, for this it is necessary to adapt the surface of the nanoparticles to the specific matrix of the coating composition or of the adhesive. The disadvantage of simple mixing in of modified nanoparticles is the dependency of the stability on the complete formulation, i.e. on all the formulation constituents. Variation of one parameter can lead here to demixing (Pilotek, Steffen; Tabellion, Frank (2005), European Coatings Journal, 4, 170 et seq.).
The use of polydimethylsiloxanes (PDMS) for the modification of lacquer systems is known from the prior art. Due to the high surface tension of PDMS, specific properties are generated, such as good surface wetting, slip resistance and an easy-to-clean surface (Reusmann in Farbe und Lack, 105, volume 8/99, page 40-47, Adams in Paintindia, October 1996, page 31-37).
In order to ensure good incorporation of PDMS and to avoid migration of the PDMS as greatly as possible, organofunctional PDMS types, such as alkyleneamine- or alkylenehydroxy-functional PDMS derivatives, are often used. Such lacquer systems are described e.g. in WO91/18954, EP-A 0 329 260 or U.S. Pat. No. 4,774,278.
Nevertheless, the amine-functional PDMS types have the disadvantage that the pot life of polyurethane systems based on these is shortened in an extreme manner because of the high tendency towards formation of urea.
The known hydroxy-functional PDMS types indeed lead to improved pot lives, but as a rule they show incompatibilities with the polyisocyanate component so that no homogeneous films can be produced and the crosslinking takes place only incompletely. As a result, free non-bonded PDMS is present in the lacquer, which in time migrates out of the coating and leads to a deterioration in the properties of the coating.
WO-A 2007/025670 discloses two-component coating compositions which comprise as a binder a polyisocyanate component in combination with a reactive component which is reactive towards isocyanate groups, in particular a polyhydroxy component. The compositions described there are suitable for the production of high quality coatings which are distinguished in particular by improved easy-to-clean properties due to a highly functional hydroxy-polydimethylsiloxane unit, while the hardness and resistance to ethanol are not improved. An improvement in the scratch resistance is not described and also was not detectable.
There is therefore still an urgent need to provide aqueous automobile clear lacquers which have an improved scratch resistance with good optical properties.
It has now been found, surprisingly, that aqueous copolymers which contain polyorganosiloxanes containing hydroxyl groups are suitable, in combination with inorganic nanoparticles, for the production of coatings having a significantly improved scratch resistance with excellent gloss and very low haze (cloudiness).
The object of the present invention was therefore to provide high quality coating compositions, in particular as automobile clear lacquers, which have an optimum gloss and haze and show an improved scratch resistance. The dispersions should furthermore be sufficiently stable to storage.